The placebo effect may contribute to the perceived and actual performance-enhancing effects of growth hormone in athletes, particularly in men, according to research presented today at the Endocrine Society's 90th annual meeting in San Francisco.

"Athletes are doping with growth hormone to improve performance, despite any firm scientific evidence that it does so," Dr. Kenneth K. Ho of the Garvan Institute of Medical Research, Sydney, Australia told the conference.

"We found that athletes who believed that they were taking growth hormone actually felt that their performance had improved and actually demonstrated improvement in physical performance, even though they were taking a placebo," Dr. Ho reported.

In general, regardless of whether the athletes guessed correctly or not about which substance they were taking, everyone showed a small improvement in performance after the study, usually about one to two percent, he said. "This is a well-recognized training effect. But among those who wrongly thought they received growth hormone, the performance enhancement was about twice that between three to five percent, Dr. Ho noted.

Dr. Ho's team evaluated the "perceived benefit" using a self-rated questionnaire and "physical performance" from tests of endurance, strength, power and sprint capacity and "it was only with power that we demonstrated a significant enhancement in those who wrongly guessed that they were taking growth hormone. This phenomenon was far greater in men than in women."

"It is clear from our study that the mind plays amazing tricks on the body, and this placebo effect has been known for many centuries. To our knowledge, this is the first study of the examination of a placebo effect in a sporting arena," Dr. Ho added.

The study was funded by the World Anti-Doping Agency and the Australian Government Anti-Doping Research Program.